1. Field of the Invention
The present invention relates to a despreading apparatus and method for a code division multiple access (CDMA) signal.
2. Description of the Related Art
CDMA systems are largely classified into a direct sequence (DS) system and a frequency hopping (FH) system. While the FH system has not been widely used, the DS/CDMA system has been adopted as a standard and is widely used in a mobile communication environment. Although a frequency division multiple access system and a time division multiple access system are also used as standards in mobile communication, the DS/CDMA system has been adopted as a standard recommendation for International Mobile Telecommunication (IMT)-2000 since the DS/CDMA system can increase the number of users that can be served more easily than the other systems.
In a CDMA system, user data is spread using a spreading code sequence and transmitted. By doing this, user data can be transmitted with a large bandwidth so that it can be robust against a multi-interference signal and multipath fading. To receive the spread signal, the same spreading code sequence used for transmission should be used for receiving. Receiving a spread signal includes the steps of searching a spreading code sequence and detecting the signal. Both steps include a procedure of despreading a signal. Despreading a signal is converting down a large bandwidth of the signal to an original small bandwidth of the signal before transmission. Accordingly, high speed signal processing is required.
A CDMA system recommended by an IS-95 standard recommendation converts a signal of 1.2288 MHz down to a signal of 9.6 KHz. Thus, the system should process data having a bandwidth 128 times wider than an original bandwidth of data before being despread. Generally, large bandwidth data requires a faster signal processing ability than small bandwidth data. To realize a software radio CDMA receiver, it is necessary to reduce the amount of calculations for despreading, which cause a bottleneck in signal processing.
Conventionally, a method of fast initial synchronization using a memory is used for fast signal processing. For example, this method is disclosed in U.S. Pat. No. 5,768,306. The memory is designed such that a received signal is stored in the memory at frequency of 1/Tc (Tc is a chip period) and is read from the memory at high frequency of K/Tc. Although data is received at a rate of 1/Tc, the data stored in the memory is read at a higher rate of K/Tc and processed so that signal processing related to a search can be performed on many signals simultaneously. Although this method is provided for searching many phase signals simultaneously in a searching apparatus operated by a fast clock, it does not reduce the amount of calculations.
Accordingly, it is desirable to reduce the amount of calculations during a real-time signal detection. If the signal detection is realized as hardware, a high clock speed is required, which generates significant heat. Generation of much heat results in large power consumption of the hardware so that using the hardware is not suitable to a mobile receiver using a CDMA signal. Accordingly, reducing the amount of calculations for the signal detection is desirable, and particularly, reducing the amount of calculations for despreading a received CDMA signal is desirable.
To solve the above problems, it is a feature of an embodiment of the present invention to provide a despreading apparatus and a method for obtaining a pilot symbol and a data symbol from a received code division multiple access (CDMA) signal using a value obtained by performing a predetermined operation on inphase and quadrature pilot pseudo noise (PN) codes when the received CDMA signal is despread.
In an effort to satisfy this and other features of the present invention, in one aspect, there is provided an apparatus for despreading a CDMA signal. The apparatus performs a predetermined operation on a received CDMA signal and first and second codes to extract a pilot symbol and a data symbol from the received CDMA signal when the first and second codes are generated internally. The apparatus includes a correlation unit for performing operations on the respective inphase and quadrature components of the received signal and the respective inphase and quadrature components of the first code and accumulatively storing the results of the operations depending on first and second selection signals which are obtained from the first and second codes. The apparatus further includes a decimator for outputting individual accumulated values after individual results of the operation are separately accumulated a predetermined number of times by the correlation unit. The apparatus additionally includes a symbol output unit for performing a predetermined operation on individual values output from the decimator to output the pilot symbol and the data symbol.
In another aspect, there is provided a method of despreading a CDMA signal, in which a predetermined operation on a received signal and first and second codes is performed to extract a pilot symbol and a data symbol from the received CDMA signal when the first and second codes are generated internally in a CDMA signal receiver. The method includes (a) performing operations on the respective inphase and quadrature components of the received signal and the respective inphase and quadrature components of the first code; (b) accumulatively storing the results of the operation a predetermined number of times depending on first and second selection signals which are obtained from the first and second codes; and (c) performing a predetermined operation on individual accumulated values after individual results of the operation are separately accumulated and outputting the pilot symbol and the data symbol.
There is also provided a method of despreading a CDMA signal, in which a predetermined operation on a received signal and first and second codes is performed to extract a pilot symbol and a data symbol from the received CDMA signal when the first and second codes are generated internally in a CDMA signal receiver. The method includes (a) grouping data of one symbol depending on the combination of the first code and the second code; (b) performing predetermined operations on the inphase component of the received signal and the inphase component of the first code with respect to the data of each group and accumulating the results of the operation during a symbol duration, and simultaneously, performing predetermined operations on the quadrature component of the received signal and the quadrature component of the first code with respect to the data of each group and accumulating the results of the operation during the symbol duration; (c) combining the accumulated results of inphase components of one group with the accumulated results of quadrature components of the other group, the groups having the same second code, thereby obtaining a plurality of partial sums; and (d) performing a predetermined operation on the partial sums to obtain the pilot symbol and the data symbol.
These and other features of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.